1. Field of the Invention
The present invention relates to a wire bonding apparatus and more particularly to a wire bonding apparatus equipped with a bonding surface level detecting means.
2. Prior Art
Conventional examples of wire bonding apparatuses which detect the level or height of the bonding surface are disclosed in Japanese Patent Application Publication ("Kokoku") No. 64-9730 and Japanese Patent Application Laid-Open ("Kokai") No. 1-31695.
In these prior art apparatuses, a tool arm that has a capillary, through which a bonding wire passes, is attached to an arm holder, and this arm holder is provided on a lifter arm in such a manner that the arm holder is pivotable. The lifter arm is mounted to a bonding head frame so that the lifter arm can move up and down or can pivot. In addition, a gap detection sensor is attached to the lifter arm. The detection sensor is installed so as to face the tool arm.
When the lifter arm is lowered, or when the lifter arm is pivoted so that the lifter arm on the capillary side is lowered, the capillary descends and is brought into contact with the bonding surface. Then, the lifter arm is further lowered or pivoted from this point, so that the gap between the tool arm and a gap detection sensor changes, and the time at which the gap distance is changed for a given amount from the initial position is taken as the time of contact of the capillary with the bonding surface.
As described above, in the prior art devices, the gap detection sensor is provided on the lifter arm, which is raised and lowered or caused to pivot, and such a gap detection sensor is installed so as to face a part of the tool arm. In other words, the sensor faces a part of the tool arm which is on the same side where the capillary is held by the tool arm. As a result, the following problems arise:
Since the sensor is on the lifter arm which is raised and lowered or caused to pivot and therefore is extremely susceptible to vibration, erroneous detections easily occur due to micro-vibrations of the lifter arm.
In addition, after the capillary has contacted the bonding surface, the gap detection sensor is lowered together with the lifter arm. At the same time, the tool arm also rotates about the bonding surface. In this case, the tool arm is moved in the direction away from the gap detection sensor; as a result, the change in the gap between the gap detection sensor and the tool arm becomes small. This causes a response delay and the inaccurate detection of the bonding surface level.
Furthermore, since the gap detection sensor in the prior art is a magnetic sensor, there is a response lag with respect to the threshold value of the sensor. This means that there is a time pass between the time when an actual contact of the capillary with the bonding surface is made and the time when the detection of the bonding surface is made by the magnetic sensor. As a result, the "sink-in" caused by the capillary that presses the wire against the bonding surface increases for the time period that corresponds to the "sink-in."